Adhesive composition components, such as base polymers, tackifiers, and waxes are customarily provided as separate components for formulation into an adhesive composition. For hot melt adhesive (HMA) application systems, the base polymer is often supplied in the form of solid pellets, which may be melted and combined with the other adhesive components.
Hot melt adhesives used in packaging applications need to give high adhesion to different substrates across a wide temperature range from subzero temperatures to temperatures above room temperature. Those adhesives are typically used on packaging lines which have to operate at very fast speed where the residence time and hence the compression times of the parts to be glued together is very short. Accordingly, in order to be successful on these lines, adhesives need to have a short set time.
Because of short and long term availability of other commonly used raw materials, it is necessary to increase the amount of polymer used in hot melt adhesive formulations. Polymers based on ethylene and its copolymers have been used in hot melt adhesives for a number of years, offering by nature a fast setting speed. Unfortunately, the high viscosity of such polymers requires the use of additional costly raw materials in rather high amounts (typically above 50%).
Polypropylene and copolymers of propylene are also attractive because of the availability and cost of the monomers. Unfortunately polymers based on propylene typically provide either good adhesion and slow setting speed or poor adhesion but good setting speed. Although functionalized waxes (for example, maleic anhydride modified waxes) can be used to improve adhesion and/or setting speed, they have a negative impact on thermal stability and compromise operations at packaging lines due to adhesive charring in the adhesive melter and application nozzle plugging.
Ethylene vinyl acetate copolymers (EVA) have been suggested as the base polymer for adhesives because of their broad compatibility with other raw materials, level of performance and cost. Unfortunately, the thermal stability of EVA based products has limitations which lead to non-desirable machine downtime for cleaning or part replacement. Furthermore, EVA materials as well as other additives used in the formulation have fluctuating availability and pricing.
Propylene-based copolymers are preferred to EVA-based polymers as their better thermal stability can reduce machine downtime. A number of references suggest how to shorten setting time of this type of polymer but unfortunately the adhesion of such formulations is limited when it comes to difficult too bond substrates, for example: varnish boards. Exemplary base polymer compositions for HMA applications and methods for producing such base polymers are disclosed in U.S. Pat. Nos. 7,294,681 and 7,524,910 and WO 2013/134038, disclosures of which are incorporated herein by reference. Various polymers described in these patents and/or produced by the methods disclosed in these patents have been sold by ExxonMobil Chemical Company as LINXAR™ polymers.
In addition, metallocene ethylene-octene based adhesives have been commercially available for many years. Those adhesives have very short set time but are lacking in good adhesion at low temperature. Metallocene propylene-hexene based adhesives have also been commercially available. While these adhesives have a reasonably short set time, they too have poor adhesion, especially at low temperature.
Accordingly, a need exists for a propylene-based hot melt adhesive that possesses both good adhesion, including that at relatively low temperatures, and fast set times, without compromise of other features such as thermal stability and processability.